Distance based shipping This module will allow you to define a set radius from 0 to any other value in your selected unit of measure imperial or metric, (base radius). This defined zone around your shipping area can have a defined cost associated with it from 0 to any other value (defined zone cost).
The unit of measure will be calculated based on the per unit of measure cost that is defined in the settings section in the admin.
Distance outside of this defined radius will incur charges based on the following formula: shipping cost = (define zone cost + ((total distance shipped - base radius) * per unit of measure cost) Distance is rounded up to nearest KM or mile. Requests are sent in SSL mode (I may change this later but would fully expect people to use SSL in the checkout section)
Add on is found at: http://addons.oscommerce.com/info/9543
cheers
Peter M